Cellulose Netting and Airtight Drywall
I was at a job site today where the cellulose contractor was finishing up and the drywall contractor was unloading his materials. The double framed exterior walls had been netted and denspacked and were ready for hanging drywall. Netting had been installed over the studs and fastened to the sides with an upholsters staple gun. The plan was to go with the airtight drywall approach and latex vapor retarder primer, and sealing the drywall to top and bottom plates as well as inside corners and window perimeters.
The drywall contractor is questioning the effectiveness of sealing to the netting rather than to exposed studs. Should we have blown in the cellulose after the drywall was up?
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Replies
Jon,
I think the netting won't interfere. Is the drywall contractor using caulk or gaskets at the top and bottom plates?
Martin,
He is using low VOC sealant.
Explain "low VOC sealant".
I'm curious. How do you DENSE pack w/ netting? I thought dense packing would bulge netting out, making sheet rocking impossible.
That's why the netting was side stapled to the studs - to keep it slightly behind the drywall surface and to increase its tension. May still require pushing on the DW, though, to get it screwed tight, which is why I prefer a closed-wall blow.
Robert,
What do you mean by "closed-wall blow"?
There are three ways to blow dense-pack cellulose:
Two open wall techniques:
1) damp spray
2) insulweb netting
One closed-wall technique (the original):
3) dense-pack (between drywall and sheathing, from either inside or outside)
Closed wall blows are significantly more expensive in new construction than netting. Not sure why you would ever consider in new construction unless costs are not a concern. The netting may bulge a bit in the middle, but if side stapled to both sides of the studs correctly, it will expand and lay flat as the drywall is installed.
Why would a closed-wall blow be more expensive when it eliminates the material and labor cost of insulweb with 3"-4" oc stapling at every stud?
No holes are required. Simply leave a 6" gap (with poly gasket) between upper and lower wall sheets, blow cellulose through slots cut in poly, fill gap with strips of 3/8" DW, tape and mud in one easy pass for an invisible seam.
In my double-wall envelopes, I simply blow from above with hoses pre-positioned in several successive stud bays and then fill the attic floor for a continuous thermal blanket with no interior or exterior penetrations required.
Wouldn't stapling the netting to the side of the studs create a gap between the insulation and the air barrier (Sheetrock)? Through Energy Star we have been told that we should not inset staple kraft faced batts due to the gap it creates. Doesn't this create the same problem? While on the topic - is dense packed cellulose an air barrier? BPI says yes if it is a min. of 3.5 lbs./cubic foot but cannot really find that in writing or any technical standards anywhere else.
Danny,
Gaps in a fiberglass batt installation create convective loops because the material is so permeable to air, in addition to decreasing the installed thickness and hence installed R-value. A dead air gap in an air-impermeable material like dense-pack cellulose does not have the same liabilities. And you're unlikely to end up with a noticeable gap after dense-packing behind insulweb.
Curiously JM Spider has a chart on their website which demonstrates that their BIBS product is as air-resistant as dense-pack cellulose. It's ironic that the industry which tried (and nearly succeeded) to destroy the cellulose industry now uses cellulose as the standard against which to measure its own performance. Both materials, they claim, reduced CFM50 by 99% over an uninsulated wall sheathed with OSB on both sides.
But the Air Barrier Association of America, which sets the standards for air barrier materials, doesn't consider cellulose to meet its requirements, which is the air-resistance of ½" drywall, or 0.004 CFM/SF @ 75P. However, cellulose by itself is never used in a building. According the JM Spider's 3rd party testing, cellulose in a wall assembly meets the AABA standards for an air barrier assembly.
The real problem with ADA and insulweb is in properly sealing the drywall to the plates and flanged outlets through the netting. A closed-wall blow is far more compatible with the ADA strategy (and less expensive).
Good information Robert - thanks. Do you have any suggestions on how to incorporate cellulose into a conditioned attic assembly?
The same way one would install cellulose in a cathedral ceiling. Create vent channels to vent the roof from soffit to ridge and isolate the ventilation air from the insulation. Frame a sub-roof with 2x4s and side-nailed gussets to create a thick enough insulation cavity with little thermal bridging. Drywall (and air barrier) the sub-roof framing and blow cellulose to min. 3 pcf density.
Makes sense - what is best product for the vent channels? I would think the styrofoam baffles would not be rigid enough for dense pack.
From this discussion, it appears to make sense to go with the closed wall approach and go with a full 8' wall studs, bottom and double top plates totalling 8'-4 1/2" allowing for a center of wall blowing slot.
For a current project, I am using Accuvent for manufactured homes which are continuous, work for both 16 and 24 inch spacing and will prevent any future roof leaks from reaching the insulation.
Styrofoam vent baffles are close to worthless. There are waxed cardboard and molded plastic vent baffles which fill the entire rafter bay. Accuvent is one of the most versatile units, and comes in several versions, including a cathedral ceiling starter and extension segments.
I make my own from 1x2 nailers and ¼" MDF.
Hi Riversong... it would seem to me that the ideal insulation baffle would allow moisture vapor to pass through to the vented space above, not stop it. For baffles I’d recommend using a product like ComfortBoard 80. Add furring strips horizontal to the joists about 1-1/2” down, then cut 2” pieces of the Rockwool to width and press into place up against the furring strips. I know this is a more expensive and labor intensive option, but it not only adds insulation in the process, it also allows any moisture to breath out into the ventilation cavity to guarantee a dry insulation cavity.
Jeff,
The comment you are replying to is ten years old. Riversong is still active in the green building community, but doesn't post on GBA.
Gotcha Malcom. No worries, I more or less posted to provide ideas to others that might come across this particular blog looking for options.
Robert, your choice of MDF surprises me...why MDF? why not plywood?
BTWay I still think GBA should put you on salary ;-)
For my last house, I got a pallet of 2'x4' MDF panels for $1 each. In that application, outside the conditioned space and in a well-vented area for easy drying, anything stiff and cheap is appropriate.
I actually used them to protect the tinted concrete floor during framing and then reused them as vent baffles.